The present invention relates to an apparatus for evaluating data representing the electrical characteristics of a combustion vessel, the combustion vessel being operable to combust a fuel.
An apparatus to evaluate data representing the electrical characteristics of a combustion vessel of the type which combusts a fuel may include a matrix of nodes arranged on an ohmic material of the combustion vessel and a wire connected to each respective node. Additionally, such an apparatus may include an electrical switch connected to each wire and to a current source.
All electrical switches have leakage currents associated with their off-states. Semi-conductor switches typically have higher leakage currents than electro-mechanical switches. One of the steps in the process of switch selection is to determine whether the off-state leakage current is sufficiently low for the application. If it is not, then either a different switch must be used or a means of reducing the leakage current must be devised. As an example, when using the four terminal ohmmeter method for measuring electrical resistivity or conductivity of a sheet material which has a varying resistivity across its area, multiple contacts to the material must be made, with the contacts typically arranged in a grid. At each measurement site, one microvolt meter or one current source connection may be made. To automate the measurement, electronic switches may be used for connecting the volt meter and current source to multiple fixed sheet contacts. In a system with hundreds or thousands of contacts, it is desirable to use only a single wire with each contact point in order to reduce wiring costs.
The single wire must carry either the excitation current, or provide a microvolt meter connection, or make no connection. If the current source switch associated with a particular wire is intended to be off and that wire is being used by the microvolt meter to sense voltage, the off-state leakage current of the switch will cause a significant voltage to develop in the wire""s finite resistance. The result will be that an error voltage is added to the desired voltage measurement. Even if double wires were used, in commercial applications such as characterization of boiler water walls, the excitation and measurement circuits would still share insulation pin resistance which could range from 3-6 mxcexa9. The magnitude of the leakage currents may be highly variable, depending on temperature, switch voltage, and the age of the switch, so the error voltages cannot be easily measured and subtracted.
It is one object of the present invention to provide an apparatus for evaluating data representing the electrical characteristics of a combustion vessel, the combustion vessel being operable to combust a fuel. According to one preferred aspect of the present invention, the apparatus includes a matrix of nodes arranged on an ohmic material of the combustion vessel, at least one wire connected to a respective node, and a switch. The switch is connected to the at least one wire and to a current source for controlling the delivery of a current to the respective node via the switch and the at least one wire and the switch has an on state and an off state, the off state allowing a leakage current through the switch. Also, the apparatus includes means for preparing a data set based on voltage data gathered from the matrix of nodes arranged on an ohmic material of the combustion vessel including calculating the resistivity of the ohmic material by iteratively imposing a known current to the matrix and measuring voltage at the nodes, each the iteration of imposing a known current resulting in a set of measured voltages. The apparatus further includes a circuit for reducing leakage currents in the switch that controls delivery of current to a load. The circuit includes a series component positioned between the switch and the load, the series component permitting delivery of current to the load through the switch and across which series component the leakage current develops a voltage and an active device connected across the series component to force the voltage across the series component to zero volts by means of feedback control, whereby the leakage current is reduced to an input current of the active device.
It is a further object of the invention to provide a new and improved circuit for reducing leakage currents in high current switches. This is accomplished by incorporating an operational amplifier and series diode between the load and the switch. The operational amplifier is configured to force the voltage across the series diode to zero volts, thereby reducing the current through the diode to zero. Since the load then xe2x80x9cseesxe2x80x9d only a connection to the series diode and a connection to the operational amplifier, the inventive circuit replaces the leakage current of the original switch with the input current of the operational amplifier. The operational amplifier typically has a much smaller input current than the typical leakage current of high current switches and also has very small input offset voltage. Since the bias currents of the amplifier are smaller than the leakage currents of the high current switch, the voltage errors associated with the amplifier""s bias currents are proportionately smaller. In addition, the input current to the operational amplifier is more stable and easily accounted for in any measurement scheme.
The circuit effectively reduces leakage currents associated with the off-state of high current switches such as power MOSFETs (metal oxide semiconductor field effect transistors). The circuit may be used with any type of semiconductor, electrical or electro-mechanical switch. The circuit has applications in many areas, including the making of multiple sheet resistivity measurements.